What do you mean by "larger sound signature"?
Anyways, some ideas:
- Koss ESP/950. They do everything the Senns can do, but better. And aren't dark. And have tighter and better extended bass. And smoother treble.
- Sony MDR-F1. Probably the most ethereal, effortless, and natural sounding mids (strings, vocals, etc) I've heard from a dynamic can - bass is lacking (super-duper open design is probably to blame), and the top-end is politely rolled off (so they aren't bright and etched like many "fast and lean" headphones), which aligns nicely next to the Senns.
- Audio-Technica ATH-W1000X. Very engaging mids, happy "v-curve" sound signature, warm overall sound, great detail and sense of space, but also somewhat colored in the lower-mid treble (like 3-5k) which may upset some.
- JVC HP-DX1000. Super-duper romantic mids (and overall tonal coloration), very warm, wet, and organic sounding. Can be absolutely heavenly with the right content, but they're also relatively bassy and don't play so nice with bass-blast music (they're not the fastest headphone in the world, so boom-boom shake the room music turns into boom-boom shake the room playback; garbage in, garbage out).
Overall the 950s are probably the "safest" choice if you like the Senns, and just want something "better." The ATs are probably the next "safest" choice because while they do get into "romantic, syrupy hi-fi sound" they aren't pushed to as much of an extreme as some headphones (like the DX1k). The Sonys, if you can find them, are very detailed and engaging through the mid-range, but they have an overall n-shaped sound signature, and may not be (depending on your taste in bass) a complete replacement option.